You will receive a piece of the Pirate set after completing Isle of Lost Skulls.

The item you receive depends on your quest rating at the end of the quest.

C rank: Cuffs / Belt

B rank: Belt / Shirt

A rank: Shirt / Hat

S rank: Any piece from the black set

As with any quest, once you have obtained every piece of the black set, you will receive a piece of the lower ranks you haven't gotten yet.

All cosmetic pieces except S rank items can be bought with gold you earn during the quest, in the lobby Gift Shop.